In general, motors and generators are substantially equivalent and differ primarily in regard to whether electrical power is applied to produce mechanical power or mechanical power is applied to produce electrical power. Because of this, the word "motor/generator" herein means both motors and generators. In both cases, the motor/generator includes a typically nonrotating component, referred to as the "stator", and a rotating component, referred to as the "rotor".
Two broad classes of motor/generators are induction motor/generators and synchronous motor/generators. An "induction motor/generator" is an asynchronous, alternating current (ac) motor/generator in which an alternating current is applied to a set of primary windings in a first member (usually, the stator) to produce a magnetic field that induces an alternating current in a set of secondary windings in a second member (usually, the rotor). Typically, the secondary windings carry only current induced by the magnetic field produced by the primary windings.
In many embodiments, the primary windings produce a magnetic field that rotates around an axis of the rotor. In such embodiments, the interaction of the magnetic field from the primary windings with the resultant induced magnetic field from the secondary windings produces a more uniform torque of the rotor about its axis. The rotational speed of this motor/generator is determined by the frequency of the power supplied by the ac current, by the configuration of the primary winding, and by the load that is rotated by this motor/generator.
"Rotating and Stationary Electrical Circuits" are the two principal circuits in an induction motor. In conventional induction motors, one winding of the two electric circuits rotates with respect to a fixed winding, and power is transferred from one circuit to the other by electromagnetic induction. Conventionally, a primary winding in one electrical circuit member (usually the stator) is connected to the power source and a secondary electrical circuit (usually the rotor) carries only current induced by the magnetic field of the primary electrical circuit during acceleration of the rotor. When the rotating electrical circuit is in synchronism with the rotating magnetic field in the air gap of the motor, the current in the rotating electrical circuit is zero.
A "synchronous motor/generator" is an ac motor/generator in which an ac current in a first member (either the rotor or the stator) produces a temporally periodic magnetic field that interacts with a temporally constant magnetic field produced by a second member (either the stator or the rotor). This constant magnetic field can be produced by permanent magnets mounted in the second member or by a dc current supplied to windings in this second member. This constant magnetic field can be produced by one or more permanent magnets mounted on the second member and/or by a dc current supplied to one or more windings, in the second member, that are connected to a dc electrical power source. The temporal period of the magnetic field from the first member should be some rational multiple of the period of rotation of the rotor.
An important parameter that characterizes a motor/generator is the maximum torque that can be produced by that motor/generator. This maximum torque is related to the product of the amplitudes of the magnetic fields produced by the stator and the rotor. It is therefore advantageous to make each of these fields as large as possible. In U.S. Pat. No. 4,176,291 entitled Stored Field Superconducting Electrical Machine and Method issued to Mario Rabinowitz on Nov. 27, 1979, the maximum torque of a synchronous motor/generator is increased by producing a particularly large dc magnetic field in the form of a magnetic field that is trapped in a superconducting cylinder.
Each cylindrical layer of superconductor is segmented to minimize hysteretic power loss. Segments in different layers are offset so that the gaps between segments in one layer do not overlie gaps in another layer, thereby reducing the effect of any fringing. A set of heaters enable the superconducting material to be placed in its nonsuperconducting state to allow the magnitude of stored field to be changed. While in this normal state, a set of field windings produce a magnetic field pattern that is trapped in this superconducting material when it is cooled sufficiently to become superconducting again. The pattern field can also be trapped in these superconducting layers by applying this pattern field with an intensity greater than the critical field H.sub.C2 while these superconducting layers are in their superconducting state. The magnetic field pattern can even be trapped while the rotor is rotating, by timing pulses to the stator windings in a manner that produces a pattern field that rotates with the rotor. The division of the superconducting layers into segments does not significantly affect the magnetic pattern field that is stored by this process.
Because the average shaft angular velocity of a synchronous motor/generator is proportional to the frequency of the applied ac current, a synchronous motor/generator cannot be turned on simply by turning on the ac current that produces its temporally periodic magnetic field. Such a motor/generator can be started by providing a mechanical torque impulse appropriate to make the rotor rotate at a rational multiple of the ac current frequency. However, if this impulse is too large or too small, the necessary frequency is not achieved.
Unfortunately, in a trapped field motor/generator, as the rotor speeds up, until it rotates in synchronism with the rotating electromagnetic field in the stator, the driving torque is much less than if the field and rotor rotate in synchronism. The driving torque is a maximum when the field and rotor are in phase. The driving torque is a minimum when the rotor is at rest. Thus, although this motor/generator exhibits excellent maximum torque, it also exhibits poor startup torque.
In U.S. Pat. No. 3,432,699 entitled Permanent Magnetic Synchronous Motor and Starting Mechanism Therefor, issued to Harry Albinger, Jr. on Mar. 11, 1969, a conventional synchronous motor is presented that includes features which insure that an initial torque impulse provides the correct amount of rotational energy to ensure that the synchronous speed of rotation is achieved. This motor includes a conventional, permanent magnet rotor having a plurality of permanent magnetic poles around the periphery thereof. A pair of stator poles receive electric timing pulses that produce the periodic magnetic field needed to make the rotor rotate.
Synchronous motors often suffer from low startup torques that prevent them from quickly accelerating to a desired rotational speed. In U.S. Pat. No. 4,814,677 entitled Field Orientation Control of a Permanent Magnet Motor issued to Allan B. Plunkett on Mar. 21, 1989, a field orientation control system for an interior permanent magnetic synchronous motor is disclosed that provides improved performance at near zero rotational speeds. In contrast to the case of dc motors in which only the amplitude of the current is controlled to control torque or speed, in an ac motor both the amplitude and phase of this current, relative to the rotor angular position, can be used to control speed and torque. Position encoders are commonly included in ac synchronous motors so that the field produced by the ac current will be produced at the appropriate rotational orientation with respect to the permanent magnetic fields in this type of motor. This patent presents a controller that produces the optimal phase without using a rotor position sensor. A current is supplied initially to one of the field windings to ensure that the rotor is initially aligned with the field produced by that winding.
The problem of low initial torque of a synchronous motor is also addressed in U.S. Pat. No. 4,830,412 entitled Starting System and Method Using a Hybrid Permanent Magnet/Induction Machine issued to Bernard A. Raad et al on May 16, 1989. This patent presents a hybrid machine, for starting a prime mover. This hybrid machine includes a squirrel cage having bars that are interposed between a set of permanent magnets. When ac power is applied to armature windings, this hybrid machine operates as a synchronous motor. When the rotor of this hybrid machine approaches synchronous rotation speed, the relative rotational velocity of the rotor and rotating field produced by the armature approaches zero, the inductively generated torque on the rotor approaches zero so that the magnetic field of the permanent magnets dominates the coupling between the rotor and the ac magnetic field from the armature. This hybrid machine thereafter operates as a synchronous motor.
Raad also discusses the following references by Mehl et al and by Cronin. Mehl et al, U.S. Pat. No. 4,481,459 discloses a permanent magnet brushless generator. Once synchronous speed is reached, ac power at the synchronous frequency of the main generator is applied to the stator windings thereof to cause the main generator to operate as a synchronous motor.
Cronin U.S. Pat. No. 4,473,752 discloses a starter-generator machine which includes a rotor-shaped stator that is fixed within a squirrel cage induction rotor. The rotor has permanent magnets on its outer circumference. Once a given rotor speed is achieved, ac power is applied to the stator windings which surround the rotor. This synchronizes the rotating magnetic field of the permanent magnets with the rotating field developed by the stator to produce maximum motive power.
In U.S. Pat. No. 4,885,494 entitled Motor and Motor Device issued to Kazuhiko Higashi on Dec. 5, 1989, a superconducting motor is disclosed that is similar to the permanent magnet motors described above, in that it also operates as a hybrid induction-synchronous motor and has a squirrel cage rotor. It differs from the above patent by Raad in that it does not have any permanent magnets.
It would be advantageous to achieve the high maximum torque of the motor presented in the patent by Rabinowitz, the increased startup torque of the hybrid machines presented in the patents by Raad and Higashi without the complexity of the motor presented in Plunkett, et al.